1. Field of the Invention
The present invention relates to a sheet conveying apparatus and an image forming apparatus such as a printer or photocopier including the same.
2. Description of the Related Art
In general, it is known that an image forming apparatus such as a laser beam printer or color photocopier includes a sheet conveying apparatus which conveys a sheet fed from a sheet feeding cassette or a manual feeding tray into the image forming apparatus.
In a sheet conveying apparatus, it is also known that the sheet is conveyed along a conveying path with the leading edge of the sheet in a skew feeding state slightly rotated with respect to the conveying direction due to such as a difference of the conveying speed by conveying rollers or a misalignment of the image transferring portion of the conveying rollers. If an image is formed with the sheet conveyed in a skew feeding state like this, it becomes a cause for forming a bad image such as an image formed on the sheet bent with respect to the sheet.
Therefore, conventionally, a sheet conveying apparatus has a skew correction apparatus which corrects for the skew feeding of a sheet to prevent the sheet from being conveyed in a skew feeding state. The skew correction apparatus includes a registration roller (downstream roller) for conveying the sheet to the transferring portion and an upstream roller for conveying the sheet to the registration roller. In addition, the sheet conveyed by the upstream roller abuts the registration roller and bends the sheet itself to form a loop therein, thereby correcting for the skew feeding of the sheet.
In the conventional skew correction apparatus, a loop is formed in the sheet by the leading edge of the sheet abutting a nip portion of the registration roller in a stop state (non-rotating state). Conveying the sheet to the registration roller by the upstream roller continues until the whole of the sheet leading edge abuts the nip portion of the registration roller, and the sheet itself is rotated to correct for the skew feeding of the sheet (See Japanese Patent Laid-Open Nos. 6-336353 and 6-345294).
However, in the conventional skew correction apparatus, if the rigidity of the sheet conveyed from the upstream roller is high and the leading edge of the sheet abuts the nip portion of the registration roller in a skew feeding state, the leading edge of the sheet encroaches on the nip portion of the registration roller. If the leading edge of the sheet encroaches on the nip portion of the registration roller like this (the state in which the sheet is squeezed into the registration roller), the sheet is made immovable by the registration roller.
Even if a loop is formed by bending the sheet in such a state, the sheet itself cannot rotate because it is made immovable by the registration roller, and the skew feeding of the sheet cannot be corrected for.
Therefore, conventionally, a sheet conveying apparatus, which is provided with a skew correction apparatus for preventing the sheet from encroaching on the nip portion of the registration roller by rotating the registration roller backward in a direction reverse to the rotation for conveying the sheet, has been devised (See Japanese Patent No. 4016621).
By rotating the registration roller backward, it is clearly possible to resolve the sheet-encroaching state even if the sheet has encroached on the nip portion of the registration roller, and it is also possible to prevent the sheet from encroaching on the registration roller.
However, in the sheet conveying apparatus described in the Japanese Patent No. 4016621, the registration roller is rotated backward a given amount regardless of the extent of the amount of encroachment of the sheet on the registration roller. Thus, if the registration roller is rotated backward a given amount even when the amount of encroachment of the sheet on the registration roller is small, there is a concern that damage such as “leading edge curling” or “leading edge folding” may occur in the sheet due to the backward rotation of the registration roller as illustrated in FIGS. 13A and 13B.
Especially when the surface of the registration roller is made of rubber material, in case of the leading edge of the sheet being curled or the like, the leading edge of the sheet is caught by the surface of the roller because the friction coefficient of the rubber surface is high, whereby the above-mentioned “leading edge curling” or “leading edge folding” is apt to occur.
Thus, it is desirable to provide a sheet conveying apparatus in which the above-mentioned problem is solved by changing the backward rotation time for rotating the downstream roller backward based on the skew amount of the conveyed sheet.